CSS Custom Properties are a relatively new standard. Work is in progress to support them in Edge, and they're shipping today in Chrome, Safari, and Firefox.

Custom property usage is fairly straightforward, just an expression like var(--prop-name) or var(--prop-name, exprAsFallbackValue). The main tricky bit is that the allowed syntax for custom property values is extremely permissive, allowing stuff like:

body {
  --js-might-read-this: if(x > 5) this.width = 10;

  --my-theme: {
    color: red;
    background: white;
  };
}

The latter style does see real world usage today to implement mixins, and is reported as a syntax error in vscode as of v1.7.1.